In order to transmit maximum allowable power, broadcast antennas should be placed at the highest possible location, as for example, on top of a hill or a tower. Such towers may be up to 2,000 feet in height in order to give the best line-of-sight to distant viewers. The transmitter which produces the high-power broadcast signal is a large structure and is necessarily located on the ground. The high-power signal must be carried from the transmitter up the tower to the feed point at the base of the antenna. VHF (54-88 MHz and 174-216 MHz) and UHF (470-890 MHz) broadcast television signals are ordinarily carried by transmission lines, which are signal conductors designed to minimize signal losses between a source and a load. At UHF frequencies, waveguides are used for carrying the signal from the transmitter to the antenna since a waveguide is particularly well suited to handle the relatively high-power signals associated with television broadcasting with a minimum of signal attenuation.
At present, for high power UHF-TV applications there are only two types of waveguides to choose from, i.e., either hollow circular waveguide or rectangular waveguide. Rectangular waveguide is advantageous since it offers a relatively wide bandwidth for the dominant mode of signal propagation and substantially no propagation of the cross-polarized dominant modes. Thus, video signal distortion due to the appearance of cross-polarized modes is minimal. However, due to the flat exterior sides of rectangular waveguide, very high lateral forces are presented to the antenna tower due to windloading, thereby requiring a more sturdy, and therefore more costly, antenna tower.
On the other hand, hollow circular waveguide has an exterior which is optimum for minimizing the effect of windloading on the antenna tower. However, unavoidable flexing and asymmetries in circular waveguide due to windloading on the waveguide itself lead to the generation of undesirable cross-polarized mode propagation and resulting video signal distortion. Cross-polarized modes effectively swing the principal plane of polarization, i.e., the dominant mode of the electromagnetic energy propagating through the waveguide, from the desired plane to a plane intermediate the desired plane and the plane of the cross-polarized mode. If an electric probe or magnetic loop is used for coupling energy from the end of the waveguide to the antenna, rotation of the polarization plane results in reduction of the power available to the antenna and also results in reflection of the remainder of the power back to the transmitter. The latter can result in undesirable picture distortion.
Thus, a broadcast system is needed in which an antenna on a tower is fed by a waveguide which has relatively low windloading while displaying stable polarization characteristics under conditions of stress and flexure.